feat(mining): "Update miner" button + dialog wiring the xmrig updater

Wires util::XmrigUpdater into the GUI:

- ui/windows/xmrig_download_dialog.h: a modal (mirrors BootstrapDownloadDialog) that drives
  the updater — Checking -> Up-to-date/Update-available -> Downloading/Verifying/Extracting ->
  Done/Failed, with a progress bar and a "verified against its published checksum" note. On
  success it persists the installed release tag to settings. Rendered each frame from App::render.
- mining_tab: an "Update miner…" button in the pool section, disabled (with a tooltip) while
  xmrig is running so a live binary is never replaced.
- settings: persist the installed DRG-XMRig tag (xmrig_version) for update detection.

Both variants build; suite passes; GUI smoke-launched without crashing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config/settings.h

@@ -300,6 +300,10 @@ public:
     bool getPoolMode() const { return pool_mode_; }
     void setPoolMode(bool v) { pool_mode_ = v; }
 
+    // Installed DRG-XMRig release tag (for in-app miner update detection); empty if unknown/bundled.
+    std::string getXmrigVersion() const { return xmrig_version_; }
+    void setXmrigVersion(const std::string& v) { xmrig_version_ = v; }
+
     // Mine when idle (auto-start mining when system is idle)
     bool getMineWhenIdle() const { return mine_when_idle_; }
     void setMineWhenIdle(bool v) { mine_when_idle_ = v; }
@@ -428,6 +432,7 @@ private:
     bool        pool_tls_       = false;
     bool        pool_hugepages_ = true;
     bool        pool_mode_      = false;   // false=solo, true=pool
+    std::string xmrig_version_;            // installed DRG-XMRig release tag (update detection)
     bool        mine_when_idle_ = false;   // auto-start mining when system idle
     int         mine_idle_delay_= 120;     // seconds of idle before mining starts
     bool        idle_thread_scaling_ = false; // scale threads instead of start/stop